在ASP.NET Core中使用托管啟動（hosting startup）程式集，實現批量註冊service

在啟動ASPNET Core時可以從外部程式集嚮應用添加增強功能。例如，外部庫可以用托管啟動（ hosting startup）實現為應用程式提供附加配置（Configuration）或服務（service）。具體實現如下： 1、實現 IHostingStartup 介面 2、標註程式集（Hos ...

在啟動ASPNET Core時可以從外部程式集嚮應用添加增強功能。例如，外部庫可以用托管啟動（ hosting startup）實現為應用程式提供附加配置（Configuration）或服務（service）。

具體實現如下：

1、實現 IHostingStartup 介面

2、標註程式集（HostingStartup）屬性。

[assembly: HostingStartup(typeof(StartupEnhancement.StartupEnhancementHostingStartup))]

3、在CreateHostBuilder中配置載入的程式集，如果多個程式集分號隔開

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    //增加外部啟動項Fap.Core.DI.ServicesInjection，初始化所有service
                    webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")
                    
                    .UseStartup<Startup>();
                });

如果阻止Hosting Startup載入，需要以下設置

 webBuilder.UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")

如果排除某些程式集的Hosting Startup載入

 webBuilder.UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, "ASSEMBLY1;ASSEMBLY2; ...")

多個程式集分號隔開。

ASPNET Core 中的 DI 沒有批量註冊service的功能。下麵我就實現一個批量註冊service的功能。

採用註解的形式，在需要註冊為service的類上進行標註。

定義一個Attribute

    [AttributeUsage(AttributeTargets.Class)]
    public class ServiceAttribute:Attribute
    {
        public ServiceAttribute(ServiceLifetime serviceLifetime)
        {
            ServiceLifetime = serviceLifetime;
        }
        public ServiceLifetime ServiceLifetime { get; set; } = ServiceLifetime.Transient;
    }

定義需要註冊的類和介面

    public interface IUser
    {
        string Get(string userName);
    }
    public interface IUser1
    {
        string Get1(string userName);
    }

    [Service(Microsoft.Extensions.DependencyInjection.ServiceLifetime.Singleton)]
    public class User : IUser,IUser1
    {
        private string s = Guid.NewGuid().ToString();
        public string Get(string userName)
        {
            return $"{s}===={userName}";
        }

        public string Get1(string userName)
        {
            return s;
        }
    }

如上，在User類上標註ServiceAttribute屬性，設置ServiceLifetime為單利模式ServiceLifetime.Singleton

接下來實現利用host startup來實現自動註冊功能。根據class上標註的service attribute 來自動註冊service

//標註程式集屬性 HostingStartup
[assembly: HostingStartup(typeof(Fap.Core.DI.ServicesInjection))]
namespace Fap.Core.DI
{
    public class ServicesInjection : IHostingStartup
    {
        public void Configure(IWebHostBuilder builder)
        {
            builder.ConfigureServices(services =>
            {
                var basePath = AppDomain.CurrentDomain.RelativeSearchPath ?? AppDomain.CurrentDomain.BaseDirectory;
                var assemblies = Directory.GetFiles(basePath, "*.dll").Select(Assembly.LoadFrom).ToArray();
                var types = assemblies.SelectMany(a => a.DefinedTypes).Select(type => type.AsType()).Where(t => t.GetCustomAttribute<ServiceAttribute>() != null).ToArray();
                var implementTypes = types.Where(t => t.IsClass).ToArray();
                foreach (var implementType in implementTypes)
                {
                    var interfaceTypes = implementType.GetInterfaces();
                    foreach (var interfaceType in interfaceTypes)
                    {
                        var attr = implementType.GetCustomAttribute<ServiceAttribute>();
　　　　　　　　　　　　　　//根據serviceLifetime來向容器中註冊
                        _ = (attr.ServiceLifetime switch
                        {
                            ServiceLifetime sl when sl == ServiceLifetime.Scoped => services.AddScoped(interfaceType, implementType),
                            ServiceLifetime sl when sl == ServiceLifetime.Singleton => services.AddSingleton(interfaceType, implementType),
                            ServiceLifetime sl when sl == ServiceLifetime.Transient => services.AddTransient(interfaceType, implementType),
                            _ => throw new FileNotFoundException("未找到此類型")
                        });
                    }
                }
            });
        }
    }
}

下麵在Host builder時設置HostStartup

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    //增加外部啟動項Fap.Core.DI.ServicesInjection，初始化所有service
                    webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")
                    
                    .UseStartup<Startup>();
                });

這樣我們在Controller中就可以使用已經自動註冊到servicecontainer中的service了。

   public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;
        private readonly IUser1 _userService1;
        private readonly IUser _userService;
        public HomeController(ILogger<HomeController> logger,IUser1 userService1,IUser user)
        {
            _logger = logger;
            _userService1 = userService1;
            _userService = user;
        }

        public IActionResult Index()
        {
            ViewBag.CC = _userService.Get("zhangsan")+_userService1.Get1("lisi");
            return View();
        }
}

除了利用IHostingStartup為應用提供服務註冊，還可以提供額外配置。

[assembly: HostingStartup(typeof(HostingStartupLibrary.ServiceKeyInjection))]

namespace HostingStartupLibrary
{
    public class ServiceKeyInjection : IHostingStartup
    {
        public void Configure(IWebHostBuilder builder)
        {
            builder.ConfigureAppConfiguration(config =>
            {
                var dict = new Dictionary<string, string>
                {
                    {"DevAccount_FromLibrary", "DEV_1111111-1111"},
                    {"ProdAccount_FromLibrary", "PROD_2222222-2222"}
                };

                config.AddInMemoryCollection(dict);
            });
        }
    }
}

在controller中就可以訪問到如上配置項

        public IndexModel(IConfiguration config)
        {
            ServiceKey_Development_Library = config["DevAccount_FromLibrary"];
            ServiceKey_Production_Library = config["ProdAccount_FromLibrary"];
          
        }

-----------------------------------------------------------------------------------------------------------

具體源碼實現：

webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")

設置了WebHostOptions中的HostingStartupAssemblies屬性，存放我們要載入的IHostingStartup的程式集。

在IWebHostBuilder 調用 Builder()返回IWebHost方法中進行調用。下麵為關鍵代碼

            _options = new WebHostOptions(_config, Assembly.GetEntryAssembly()?.GetName().Name);
　　　　　　　//沒有設置阻止載入webBuilder.UseSetting( WebHostDefaults.PreventHostingStartupKey, "false")
            if (!_options.PreventHostingStartup)
            {
                var exceptions = new List<Exception>();

                // 執行 hosting startup assemblies
                foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
                {
                    try
                    {
                        var assembly = Assembly.Load(new AssemblyName(assemblyName));

                        foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                        {
　　　　　　　　　　　　　　　　　//實例化自定義的hostingStartup
                            var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType);
　　　　　　　　　　　　　　　　　//調用Configure方法，執行我們自定的邏輯
                            hostingStartup.Configure(this);
                        }
                    }
                    catch (Exception ex)
                    {
                        // Capture any errors that happen during startup
                        exceptions.Add(new InvalidOperationException($"Startup assembly {assemblyName} failed to execute. See the inner exception for more details.", ex));
                    }
                }

                if (exceptions.Count > 0)
                {
                    hostingStartupErrors = new AggregateException(exceptions);
                }
            }

_options.GetFinalHostingStartupAssemblies()方法代碼如下：

   public IEnumerable<string> GetFinalHostingStartupAssemblies()
        {
　　　　　　　//返回HostingStartupAssemblies中排除掉HostingStartupExcludeAssemblies的程式集
            return HostingStartupAssemblies.Except(HostingStartupExcludeAssemblies, StringComparer.OrdinalIgnoreCase);
        }